Multifunctional cerium-based nanomaterials and methods for producing the same

ABSTRACT

Embodiments relate to a cerium-containing nano-coating composition, the composition including an amorphous matrix including one or more of cerium oxide, cerium hydroxide, and cerium phosphate; and crystalline regions including one or more of crystalline cerium oxide, crystalline cerium hydroxide, and crystalline cerium phosphate. The diameter of each crystalline region is less than about 50 nanometers.

RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 14/307,301 filed Jun. 17, 2014, which is hereby incorporated byreference in its entirety. This application also claims benefit under 35U.S.C. 119 (e) of U.S. provisional Application No. 62/836,008 filed onJun. 17, 2013, which is hereby incorporated by reference in itsentirety.

GOVERNMENT GRANTS

This work was supported by the Department of Energy National EnergyTechnology Laboratory under award numbers DE-EE0005660.

BACKGROUND

Use of AZXX Mg alloys is widespread as they are ideal alternatives to Alalloys and steel due to their lightweight nature and the correspondingmechanical properties (e.g. specific stiffness, specific strength).Among the AZXX series, AZ31, AZ61 and AZ91 are the major commercialalloys. However, the high chemical reactivity and poor corrosionresistance of Mg alloys have limited their use. In general, increasingAl content in AZXX alloys leads to better corrosion resistance butdecreases ductility due to the formation of the brittle Mg₁₇Al₁₂intermetallic phase (β). In particular, AZ91 has been extensivelyinvestigated due to its higher yield and ultimate tensile strengths aswell as better corrosion resistance than AZ31 and AZ61. In comparison,AZ31 can be used to form more complex shapes due to better ductility,but has high chemical reactivity associated with the low Al content ofabout 3 weight percent (wt. %).

Chemical conversion coatings (CCs) are widely used as the initial layerof a coating system for protection of Mg alloys. Cerium-based conversioncoatings (CeCCs) are capable of providing excellent corrosion resistancefor high strength Mg and Al alloys when proper processing parameters areused.

Bulk CeO₂ has a stable cubic structure (fluorite type, space group Fm3m)from room temperature to the melting point (about 2500° C.). CeO_(2-x)has a cubic fluorite structure up to x≈0.2 but additional structuressuch as rhombohedral, monoclinic, and triclinic are possible at0.2<x<0.3. The electronic structure of cerium gives its compoundsunusual physical, chemical and electrochemical properties. Cerium existsin two oxidation states, Ce(III) when the 4f orbital is occupied withone electron (4f′) and Ce(IV) when unoccupied (4f⁰).

Cerium based oxides, such as oxygen deficient CeO_(2-x), aretechnologically important because the Ce(III)/Ce(IV) couple may undergorapid reduction-oxidation (redox) cycles at particular environmentalconditions. The reduction mechanism from Ce(IV) to Ce(III) species incerium oxides is not known, but Ce(III) is favored in oxygen-deficientatmospheres at elevated temperatures (e.g., 200-1000° C.). Thus, adramatic alteration of environmental conditions is often necessary toeffect a Ce(IV)/Ce(III) redox cycle.

SUMMARY

Embodiments relate to a cerium-containing nano-coating composition, thecomposition including an amorphous matrix including one or more ofcerium oxide, cerium hydroxide, and cerium phosphate; and crystallineregions including one or more of crystalline cerium oxide, crystallinecerium hydroxide, and crystalline cerium phosphate. The diameter of eachcrystalline region is less than about 50 nanometers.

Embodiments also relate to a method for producing a composition. Themethod includes immersing a metal substrate in a cerium-containingaqueous bath having a cerium-containing conversion coating on thesubstrate, spontaneously depositing a nano-coating on the substrate. Thenano-coating includes an amorphous matrix including one or more ofcerium oxide, cerium hydroxide, and cerium phosphate; and crystallineregions including one or more of crystalline cerium oxide, andcrystalline cerium hydroxide. The method also includes immersing thenano-coated substrate in a phosphate-containing solution sufficient toconvert the deposited nano-coating to a phosphated nano-coatingincluding:

-   an amorphous matrix including one or more of cerium oxide, cerium    hydroxide, and cerium phosphate. The crystalline regions include one    or more of crystalline cerium oxide, crystalline cerium hydroxide,    and crystalline cerium phosphate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram illustrating a method for preparing anano-structured composition, according to one embodiment of thedisclosure.

FIG. 2A shows a cross-sectional view of a prepared substrate, accordingto one embodiment of the disclosure.

FIG. 2B shows a top view of a surface of a prepared substrate, accordingto one embodiment of the disclosure.

FIG. 3A shows a top view of a surface of a prepared substrate, accordingto one embodiment of the disclosure.

FIG. 3B shows a cross-sectional view of a prepared substrate, accordingto one embodiment of the disclosure.

FIG. 4A shows a top view of a surface of a prepared substrate, accordingto one embodiment of the disclosure.

FIG. 4B shows a cross-sectional view of a prepared substrate, accordingto one embodiment of the disclosure.

FIG. 5A shows a top view of a surface of a deposited composition,according to one embodiment of the disclosure.

FIG. 5B shows a cross-sectional view of a composition deposited on asubstrate, according to one embodiment of the disclosure.

FIG. 6A shows a top view of a surface of a post-treated composition,according to one embodiment of the disclosure.

FIG. 6B shows a cross-sectional view of a post-treated compositiondeposited on a substrate, according to one embodiment of the disclosure.

FIGS. 7A and 7C show top views of a deposited composition, according toone embodiment of the disclosure.

FIGS. 7B and 7D show top views of a post-treated composition, accordingto one embodiment of the disclosure.

FIG. 8 shows electrochemical impedance spectra for a substrate andcomposition deposited on the substrate at several stages of deposition,according to one embodiment of the disclosure.

FIG. 9 shows a UV-visible spectra of a composition with and withoutsunlight exposure, according to one embodiment of the disclosure.

FIG. 10A shows a top view of a surface of a composition without exposureto sunlight, according to one embodiment of the disclosure.

FIG. 10B shows a top view of the surface of the composition withexposure to sunlight, according to one embodiment of the disclosure.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide an understanding of the invention. One skilled in the relevantart, however, will readily recognize that the invention can be practicedwithout one or more of the specific details or with other methods. Inother instances, well-known structures or operations are not shown indetail to avoid obscuring the invention. The present invention is notlimited by the illustrated ordering of acts or events, as some acts mayoccur in different orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with the present invention.

As used herein, the recited terms have the following meanings. All otherterms and phrases used in this specification have their ordinarymeanings as one of skill in the art would understand.

As used herein, “ambient conditions” refers to conditions at one or moreof about standard temperature and about standard pressure (STP), orabout 298.15 K (25° C., 77° F.) and about 100 kPa (14.504 psi, 0.987atm), respectively. One of skill in the art will recognize that ambientconditions can vary based on factors such as elevation, season of theyear, and global latitude, and such variations can readily be realizedand considered when reviewing this disclosure. In particular, “ambientconditions” can refer to conditions near STP, wherein no significantheat or pressure variations are required to effect a desired effect(e.g., a redox reaction).

As used herein, “morphology” refers to the arrangement of one or moreelements, on or more molecules, or combinations thereof. In particular,“morphology” can refer to surface characteristics of nano-scalecoatings. For example, “morphology” may refer to cracking, or otherstructures (i.e., spheres or nodules) present on a surface.

As used herein, “nano”, “nanocrystalline”, “nanocrystal”, “nanoscale”,“nanostructured” refer to sizes, characteristics, and structures havingnano dimensions, typically within the range of 0.01 nm to 1000 nm.

As used herein, “sunlight” refers to a broad spectrum of lightwavelengths, including the visible light spectrum from about 400 nm toabout 700 nm, but also including, for example, the Infra-Red spectrumfrom about 700 nm to about 1 mm and the ultraviolet spectrum from about10 nm to about 400 nm. Sources of sunlight can include the sun, andother sources, for example, LEDS, and incandescent bulbs.

Unless otherwise specified, “cerium” and “Ce”, as used herein, can referto one or both of trivalent cerium (Ce(III)) or tetravalent cerium(Ce(IV)).

Nanostructured cerium oxides have promoted new applications such asphotocatalysis, but the wide band gap of CeO₂ (i.e., 3.2 eV) limitsabsorption to UV light sources. A suitable water splitting photocatalystwill have a band gap wider than about 1.7 eV; have conduction band andvalence band potentials that surround both H₂ and O₂ evolutionpotentials; be stable in various operating environments (e.g., low orhigh pH); be able to efficiently convert photons to electron-hole pairs;and allow for rapid migration of electron-hole pairs to the reactionsurface such that charge recombination is sufficiently minimized.Corrosion resistance is critical for applications such as photocatalysisbecause charge transferred by the catalysts exacerbatesself-degradation.

Provided herein are compositions comprising cerium-containingnanocrystals embedded in amorphous matrices. An amorphous matrix is astructure having no long range order or crystalline aspects, or wherethe lattice d-spacing becomes highly inconsistent. Particularly, anamorphous matrix can be characterized by having little or noidentifiable lattice fringes. Crystalline aspects can include an orderedatomic pattern or lattice structure extending in three spatialdimensions. Cerium-containing nanocrystals can include trivalent andtetravalent cerium present as one or more of oxides (e.g., Ce₂O₃, CeO₂,and CeO_(2-x) where 0.001<x<0.2), hydroxides (e.g., Ce(OH)₃), andphosphates (e.g., CePO₄). The nanostructured cerium-based compositions,such as Ce oxides, provided herein have properties which arebeneficially different than bulk Ce materials and other Ce materialsknown in the art (e.g., larger crystals). In particular, thenanostructured Ce compositions provided herein allow for aCe(IV)/Ce(III) redox couple which is reversible in both directions(e.g., oxidation from Ce(III) to Ce(IV), and reduction from Ce(IV) toCe(III)). In particular, the Ce(IV)/Ce(III) redox couple can bereversible at ambient conditions.

Further, the Ce(IV)/Ce(III) redox couple can be reversible at constantenvironmental conditions. Environmental conditions can include one ormore of temperature, pressure, sunlight, and humidity. Suchcompositions, for example nanocrystalline CeO₂ and CeO_(2-x), have heatsof reduction which can be less than one-half of bulk crystallinesamples. For example, in some embodiments, compositions disclosed hereinmay have a heat of reduction of about 2 eV, as compared to the heat ofreduction of about 4 eV for bulk CeO₂. Reducing crystal size, forexample crystal diameters less than about 50 nm and particularly lessthan 15 nm, can generally lead to lower heats of reduction. Thecalculated band gaps of these nanostructured CeCCs enable absorption oflight wavelengths in the visible light spectrum. Therefore,photocatalytic reduction applications of Ce(IV) to Ce(III) can be morereadily utilized in a wider range of conditions (e.g. ambientconditions). In addition, the nanostructured CeO_(2-x) polycrystalsprovided herein have shown reduced grain boundary resistance and higherelectronic conductivity than bulk cerium materials.

The compositions described herein exhibit self-healing behavior, in partdue to the ready reduction of Ce(IV). Self-healing behavior includes oneor more of reducing surface cracks, and improving general compositioncharacteristics. For example, an improved composition characteristiccould be increased electric conductivity or corrosion resistance.Self-healing behavior can occur when compositions described herein areexposed to one or more of ambient conditions, sunlight, and moisture.The decrease in cracks, and expansion of the lattice and volume changescan be attributed to, among other things, reduction of Ce(IV) and thelarger ionic radius of Ce(III) compared to Ce(IV) ions and an increasein oxygen vacancies. Through photocatalytic reduction, the percentage ofCe(III) species out of all Ce species present in the CeCC can beincreased by at least 13%, at least by 20%, or at least up to 33%. Insome embodiments changes in oxidation state and lattice expansion ofcerium oxides nanoparticles can correlate with particle size; thesmaller the particle size, for example particles less than 15 nm, thelarger the fraction of Ce(III) species and the larger the latticeparameter. In many embodiments, the CeO₂ and CePO₄ nanocrystals embeddedin an amorphous matrix evolve to a mixture of larger CeO₂ and CePO₄nanocrystals in the presence of sunlight. In some instances, thenanocrystals can increase in size by at least 1.5 about times, by atleast about 2 times, or at least about 2.5 times.

In other embodiments, the Ce(IV)/Ce(III) redox couple is reversible byone or more of deprivation of sunlight, deprivation of moisture, andtime. In some such environments, the environmental conditions candetermine the rate and extent of the redox couple reversibility. Forexample, a photo-reduced composition can exhibit partial or fulloxidation of reduced Ce compounds after 1 week, after 2 weeks, after 3weeks, or after more than 4 weeks of sunlight deprivation. Reversal ofCe(IV) reduction can be indicated by one or more of color change (e.g.,from translucent to yellow) and reappearance of cracks.

In some embodiments, to achieve Ce(IV) reduction the compositions areideally exposed to sunlight in at least 45% relative humidity, at least50% relative humidity, at least 55% relative humidity, at least 60%relative humidity, at least 65% relative humidity, or at least 70%relative humidity. Water can be an important electron donor for Ce(IV)to Ce(III) reduction.

In some embodiments, photo-induced color change is a functional featureof the compositions and methods provided herein. In other embodiments,color changes can indicate the ratio of Ce(III) to Ce(IV) species;orange to dark yellow coatings can indicate a more Ce(IV) richcomposition whereas pale yellow to translucent colors can indicate amore Ce(III) rich composition.

The morphological changes exhibited by the compositions provided hereinto fewer cracks and smaller nodule sizes during sunlight exposureindicate a change in CeCC structure. The exhibited self-healing behaviorof coatings during sunlight exposure is beneficial for any furtherphoto-induced process because corrosion can be inhibited and, ingeneral, the integrity of the coating and the substrate can beprotected.

Due to these characteristics, cerium oxide containing compositions asdisclosed herein can be used, for example, in photocatalysis, corrosionresistant coatings, chemical degradation, UV protective coatings orsunscreens, solid oxide fuel cells, sensors, and other solid stateelectrochemical devices. For example, these cerium-containingcompositions can be used as photocatalysts for toluene oxidation, azodyedegradation, and water decomposition or splitting. In some embodiments,provided herein are nano-scale Ce-based compositions having band gapssuitable for photocatalysis by light sources outside the UV spectrum.Such materials are advantageous as they allow for more simplified andefficient photocatalytic applications which do not require photovoltaicand/or electrolytic systems. Specifically, band gaps (E_(g)) can beadjusted between about 2 to about 4 eV and the ratio of theCe(III)/Ce(IV) species between 0.1 to 1.0, or more preferably between0.2 to 0.9. As the Ce(III)/Ce(IV) ratio increases the band gap increasesalong with corrosion protection. Smaller ratios decrease the band gapand can be more beneficial for higher efficiency absorption of thesunlight spectra.

FIG. 1 shows an illustrative method 100 for preparing the compositionsprovided herein, and can comprise one or more of selecting a substrate111, preparing 110 a substrate 111, depositing 150 a CeCC 151 on asubstrate 111, and post-treating 160 the CeCC 151. Preparing 110 cancomprise one or more of grinding/polishing 120 the substrate 111, acidtreatment 130, and alkaline treatment 140. While suitable pre-treatment,deposition, and post-treatment techniques are provided and deemedadvantageous and efficacious over others known in the art, it is alsothe particular compositions themselves which provide the advantages towhich this disclosure is directed. Accordingly, the particularpre-treatment, deposition, and post-treatment techniques describedherein are presented both for their novelty and as enablement, and thenovel compositions presented herein are not intended to be limitedthereby.

Suitable substrates 111, in general, will be more reactive or havenegative electromotive forces, while more noble substrates, in someembodiments, are less preferable. In some embodiments, aluminum,magnesium, or combinations thereof are suitable substrates. Inparticular, AZXX and AMXX series alloys are suitable, including themajor commercial alloys AZ31 (3 wt. % Al, 1 wt. % Zn, 0.2 wt. % Mn, andthe balance Mg), AZ61 (6 wt. % Al, 1 wt. % Zn, 0.2 wt. % Mn, and thebalance Mg) and AZ91 (9 wt. % Al, 1 wt. % Zn, 0.2 wt. % Mn, and thebalance Mg), and AM30 (3 wt. % Al, 0.2 wt. % Mn, and the balance Mg) andAM60 (3 wt. % Al, 0.2 wt. % Mn, and the balance Mg). These alloys aregenerally available in rolled sheet form for shaping into body panelsand the like. The nominal compositions as given are stated with theunderstanding that, in some embodiments, minor amounts of impurities mayexist. Manganese is often added to control iron impurities in AZXXalloys. Embodiments containing Mn may comprise manganese-based compoundssuch as Al₆Mn or Al₂Mn. Other alloys including AA 7075-T6 (5.6 wt. % Zn,1.6 wt. % Cu, 0.3 wt. % Cr, 2.5 wt. % Mg and the balance Al), and AA2024-T3 (0.6 wt. % Mn, 4.5 wt. % Cu, 1.5 wt. % Mg and the balance Al),and the like, are also suitable substrates for the compositions andmethods provided herein. The above alloys can, in some embodiments, besuitable in Al Clad form. Other substrates having the same or similarcomposition and/or microstructure can similarly be suitable. Generally,more reactive substrates having negative electromotive forces

Surface preparation 110 prior to CeCC deposition 150 must be appropriatefor the process as inadequate treatment can produce inhomogeneous and/ornon-adherent coatings. For example, the presence of the β phase at thesurface of AZ91D alloys can be desirable for good corrosion properties,but it may be detrimental for uniform coating deposition. Appropriatecontrol of the β phase can be obtained by a combination of acid andalkaline surface preparation steps which influence the electrochemicalnature of the surface for subsequent coating deposition. Surfacepreparation can comprise one or more of grinding/polishing, acidtreatment, and alkaline treatment.

The morphology and photocatalytic and other properties of thecompositions described herein may be related to one or more of thesubstrate surface preparation, and the microstructure of the finalcoating. For example, the type of deposition substrate (e.g. AZ91D Mgalloy), and/or the substrate preparation process, including both thetype and order of preparation steps, can affect the properties of acoating deposited on the substrate. These results can be substratespecific. In some cases, a preferred preparation method for an AZ91D Mgalloy substrate comprises an alkaline cleaning, or an acid cleaningfollowed by an alkaline cleaning. A suitable pretreatment method forAZ31 and AZ91 Mg alloys can comprise surface grinding followed by acidand then alkaline immersion cleaning.

Surface grinding and/or polishing 120 can remove surface contaminantsfrom the fabrication process and the thick oxide/hydroxide layers thatformed naturally on the surface. Removing surface contaminants can becritical as they can create active cathodic sites which serve toexacerbate corrosion and may also affect subsequent coating processes.Grinding a Mg and Al substrate provides a surface 112 comprising Mgoxides and hydroxides and Al oxides and hydroxides. Grinding can beaccomplished by methods known in the art, such as by silicon carbidemedium. The topography of a ground substrate, in some cases, canperpetuate to the final CeCC.

Pretreatment cleaning and other methods can change the composition of asubstrate's surface. For example, pure Mg and AZ80 when immersed inwater for 48 hours can form a Mg(OH)₂ outer layer on the top of an innerMgO-rich layer. In general, a combined layer about 700 nm thick can formon a pure Mg substrate while a combined layer about 200 nm thick canform on a AZ80 substrate. Similar layers can form on pure Mg substrateswhen exposed to alkaline solutions (e.g., 1MNaOH) for 48 hours, butbecause Mg passivates in basic solutions and thinner layers aretypically formed (e.g., <400 nm).

Acid treatment 130 can further remove surface contaminants and create amore homogenous and active alloy surface thereby promoting thicker andmore adherent and protective coatings as compared to coatings depositedon polished or as-received alloy surfaces. In some embodiments, acidtreatment 130 can be performed until all native oxide layers areremoved. Acid treatment 130 will form a new layer 131 having acomposition different from the substrate 111.

Acid treatment 130 often favors oxide formation over hydroxideformation, for example, by forming up to about 85% oxides. Acidtreatment 130 can change the surface composition and morphology relativeto the underlying Mg alloy, for example, by increasing the atomicpercent (at. %) of Al in a newly formed layer by up to about 1.5 times,up to about 2 times, up to about 2.5 times, up to about 3 times, orgreater than about 3.5 times that of the Mg alloy. Increasing Al contentin the newly formed layer 131 can increase corrosion resistance as Alrich oxides and hydroxides are typically more stable in a wider range ofenvironments (e.g., pH variations, reactive species, etc) than Mg oxidesand hydroxides. Accordingly, acid solution concentrations and/orapplication durations can be increased, individually or cooperatively,to increase Al content in the newly formed layer 131.

For example, Al content in the newly formed layer 131 can be increasedup to about 2 times, up to about 3 times, up to about 4 times, up toabout 5 times, or greater than about 5 times that of the Al content inthe substrate. Acid treatments 130 can also be tuned to adjust Alcontent with respect to Mg content in the newly formed layer 131. Forexample, the Mg/Al ratio in the newly formed layer 131 can be decreasedto about 20, to about 15, to 10, to about 5, or to less than about 5 theMg/Al ratio of the substrate. In most embodiments, the Mg and Al speciescomprising the newly formed layer 131 are predominately oxides.

In some embodiments, acid treatment 130 duration and/or acid treatmentconcentrations can be tuned to achieve a desired thickness for the newlyformed layer 131. The newly formed layer can be, for example, about 5 nmto about 50 nm thick, although thicker layers are practicable providedadherence and uniformity aspects of the layer are maintained.

Acid treatment 130 can be performed using sulfuric acid, nitric acid,hydrofluoric acid, or other acids known to those of skill in the art.Different acids can be preferential for different Mg alloys. Forexample, nitric acid treatment on AZ91D alloys can preferentially etchthe Mg/Al phase boundaries and promote a relatively homogeneousdissolution of the Mg phase. In some embodiments, after mechanicalprocesses prepare a deposition substrate the substrate can be immersedin a 1 wt. % HNO₃ aqueous solution for 30 seconds. When no mechanicalprocesses are used to prepare Mg alloys for coatings, acid treatmentscan be performed at higher concentrations and longer immersion times toremove contaminants and the native oxide layer. In some embodiments,acid treatment 130 is performed by spraying and/or immersing a substratein an aqueous bath. Acid treatment 130 can be followed with a waterwash, for example a deionized water wash.

Alloys, such as Mg alloys, treated with acid can be further enhanced byalkaline treatment 140 to modify a substrate or the layer 131 formedduring a prior acid treatment. Alkaline treatment 140 can fully orpartially change the composition and/or morphology of the layer 131formed during acid treatment 130. For example, a portion of layer 131can be altered via alkaline treatment 140 to create an altered layer141, and a portion of layer 131 remains present between altered layer141 and substrate 111. In some embodiments, a substrate can undergoalkaline treatment 140 without a prior acid treatment 130. In suchembodiments, alkaline treatment can form a layer 141 having acomposition and/or morphology different from the substrate 111 andsuperficial native layer, if present.

Alkaline treatment 140 promotes selective Al dissolution from thesurface layer 131 or the substrate 111, and can form a porous layer 141which is predominantly comprised of Mg and Al hydroxides. For example,selective dissolution of Al rich phases in AZ91D occurs when thecombination of an acidic solution is used to expose the aluminum phaseand a subsequent alkaline solution is used to dissolve that phase. Theincreased surface area provided by Al dissolution can enhance adhesionand corrosion resistance of subsequent conversion coatings. Therefore,the concentration and contact time of alkaline treatment 140 can betuned to create an increasingly porous surface. Additionally, prior acidtreatment 130 can be cooperatively tuned to modify the Al content of thelayer 131 formed during acid treatment 130 to provide more or less Alavailable for dissolution.

In many embodiments, the Al content of the alkaline-modified layer 141has greater Al content than the substrate 111. In such embodimentscomprising a prior acid treatment 130, the Al content is typically lowerin the alkaline-modified layer 141 than the layer formed during acidtreatment 131. In other embodiments, the alkaline-modified layer 141 hasless Al content than the substrate.

Alkaline treatments 140 are further useful in controlling the hydroxidespecies present in layer 141. The hydroxide species are important forpromoting spontaneous deposition of CeCCs. When metal substrate,particularly Mg substrates or Mg alloy substrates, panels are immersedinto the acidic CeCC deposition solution, the OH— groups associate withthe metallic hydroxides. Breakdown of the hydrogen peroxide speciesraises the pH near the surface of the metal substrate, enhancingspontaneous deposition of cerium species. Spontaneous deposition isdeposition which occurs, for example, without an applied electricpotential.

Accordingly, the concentration of the alkaline solution and theapplication time thereof to a surface layer can be modified to achieve adesired surface composition. In some embodiments, a suitable aqueousalkaline solution comprises about 5 wt. % of Na₂SiO₃.5H₂O. In otherembodiments, NaOH can be used, particularly with Al alloys and Mg alloys(e.g., AZ31, AZ61, AZ91, AM60, and AM30). Another example of a suitablealkaline cleaner is an aqueous solution of sodium carbonate containingabout 5 percent by weight of sodium carbonate. The order and means ofapplication of aqueous acid cleaning and alkaline cleaning is a matterof choice, and the application and/or immersion times can similarly bemodified. This step can be followed with a water wash, for example adeionized water wash, before the CeCC 151 is deposited.

A CeCC 151 can be deposited 150 on a substrate 111 by immersion in anaqueous cerium-containing bath. In some embodiments a layer 152 ispresent between the deposited CeCC 151 and the substrate 111. Layer 152can comprise one or more of layer 131 and layer 141. In some embodimentslayer 152 is not present because one or more of an acid treatment 130 oran alkaline treatment 140 was not performed before CeCC deposition 150.In other embodiments, layer 152 is not present because CeCC deposition150 fully altered the composition and/or morphology of a prior layerpresent on the surface of a substrate 111.

Elemental Ce can comprise about 0.1 wt. % to about 2.0 wt. % of theaqueous bath. Suitable sources of Ce include CeCl₃-7H₂O andCe(NO₃)₃-6H₂O. Typically depositing CeCCs 151 on metal substrates 111,such as AZ31 Mg alloys, can require long immersion times (e.g., >30 min)for suitable purposes, such as providing effective corrosion inhibitionor achieving desired CeCC 151 thicknesses. The aqueous bath can furthercomprise an accelerator, such as an organic accelerator. For example,small additions of hydrogen peroxide (e.g., <8 wt. % H₂O₂) to the CeCCsolution can greatly reduce the deposition time, in addition toaffecting the Ce(III) to Ce(IV) ratio. Anti-bubbling agents can also beadded to improve CeCC 151 quality by one or more of reducing gas bubblegeneration during deposition, altering the deposition kinetics, andmodifying nanocrystal size. An example of an anti-bubbling agent areorganic and synthetic gelatins, which can be added in small amounts fromabout 10 ppm to about 1,000 ppm, or from about 400 ppm to about 800 ppm.In some embodiments gelatin can allow for a more uniform and adherentCeCC 151. In some embodiments, pH of the aqueous solution is adjustedusing acid, such as HCl, to suitable levels. For example, a pH betweenabout 1.0 and about 3.5, or a pH between about 2.3 to about 2.7 can besuitable.

The cerium present in the CeCC comprises both Ce(III) and Ce(IV)species. A deposited CeCC 151 can be about 100 nm to about 1,000 nmthick, about 200 nm to about 800 nm thick, or about 300 nm to about 600nm thick. For example, a CeCC 151 can be about 400 nm thick. Depositionthickness can alter the ratio of Ce(III) to Ce(IV) and the CeCC 151 bandgap.

In general, the CeCC 151 comprises nanocrystals 155 no larger than 50 nmin diameter embedded in an amorphous matrix 156. One or both of thenanocrystals 155 and the amorphous matrix 156 comprise cerium in atleast an oxide form, a hydroxide form, and a phosphate form. Ideally,the CeCC 151 comprises Ce-containing nanocrystals 155 about 1 nm toabout 20 nm in diameter, about 2 nm to about 15 nm in diameter, or about3 nm to about 10 nm in diameter. In some embodiments the CeCC 151 willcomprise Ce-containing nanocrystals 155 less than about 5 nm indiameter, less than about 4.9 nm in diameter, less than about 4.8 nm indiameter, less than about 4.7 nm in diameter, less than about 4.6 nm indiameter, less than about 4.5 nm in diameter, less than about 4.4 nm indiameter, less than about 4.4 nm in diameter, less than about 4.3 nm indiameter, less than about 4.2 nm in diameter, less than about 4.1 nm indiameter, or less than about 4.0 nm in diameter.

The unique surface nano-morphology and composition comprising one ormore of Ce oxides, hydroxides, and phosphates, or particularly two ormore of Ce oxides, hydroxides, and phosphates, or comprising Ce oxides,hydroxides, and phosphates are preferably achieved by the spontaneousdeposition methods as described herein. Other deposition methodsincluding electrocoating, sputtering, and MOCVD, known in the art can besuitable provided they are able to achieve the particular compositionsand morphologies as described herein.

In some embodiments a CeCC 151 is post-treated 160 by immersion in anaqueous phosphate-containing bath. In some embodiments a layer 162 ispresent between the deposited post-treated CeCC 161 and the substrate111. Layer 162 can comprise one or more of layer 152 and CeCC 151 asdiscussed above. In some embodiments layer 162 is not present becauseone or more of an acid treatment 130 or an alkaline treatment 140 wasnot performed before CeCC deposition 150. In other embodiments, layer162 is not present because CeCC post-treatment 160 fully altered thecomposition and/or morphology of a prior layer or layers present on thesurface of a substrate 111.

Elemental P can comprise about 0.5 wt. % to about 0.8 wt. % of theaqueous post-treatment bath. Suitable sources of P include NaH₂PO₄ andNH₄H₂PO₄. Post-treatment 160 can also reduce cracks present in a CeCC151 and reduce the size of cerium-containing nodules deposited during aprior CeCC coating 150. For example, Ce-containing nanocrystal diameterscan be less than about 50 nm, less than about 25 nm, less than about 10nm, about 5 nm, or less than about 5 nm. Lattice spacing can be about0.32 nm. Similar to the deposited CeCC 151, the post-treated CeCC 161comprises nodular Ce-oxide nanocrystals 155 and Ce phosphatenanocrystals 165 embedded within an amorphous matrix 166. This layer cancomprises a combination of {102} hexagonal structure CePO₄.H₂O and {111}cubic structure CeO₂ and Ce₂O₃. In some embodiments, Ce hydroxidenanocrystals are additionally present within the amorphous matrix 166.

Phosphate post-treatment 160 of a CeCC can alter the microstructure andmorphology of the coating compounds and can effect various responses andevolutions of a CeCC to ambient and other environments. Particularly,the overall ratio of Ce(IV) species to Ce(III) species in both theamorphous and crystalline regions can be decreased, and ultimately theband gap of the resulting CeCC can be manipulated thereby. Prior tophosphate post-treatment 160, the amorphous matrix 156 generally has alower Ce(IV) to Ce(III) ratio than the nanocrystalline phases. However,this does not necessarily hold true after phosphate post-treatment 160as the Ce(III) species can increase at a higher rate in the crystallineregions 155 and 165 than the amorphous matrix 166. The increase inCe(III) species, in the crystalline and amorphous matrix regions, is atleast in part due to the formation of CePO₄. Therefore, manipulatingpost-treatment times and/or the concentration of post-treatmentsolutions can be used to tune the band gap of the post-treated CeCC. Theband gap can be further manipulated by exposure to sunlight.

The minimum potential difference (voltage) needed to split water is1.23V at 0 pH and ambient conditions, corresponding to a minimumtheoretical light wavelength of about 1000 nm. Provided herein aremethods and compositions having suitable electronic structures andoptical properties such that reduction of Ce(IV) species is promotedonly by one or more of ambient conditions and direct sunlight exposure.In particular, the compositions provided herein have conduction and thevalence band potentials above and below the hydrogen and oxygenevolution reactions, wherein each range is independently orcooperatively tunable based on, for example, composition pre-treatment,deposition, and post-treatment techniques as described herein, andexposure to one or more of sunlight and ambient conditions.

The cerium-based compositions deposited on Al and Mg alloys as describedherein are suitable for this purpose and are able to absorb visiblelight, UV light, and more energetic wavelengths, due to the tunable bandgap range of about 2 eV to about 4 eV. In particular, some as-depositedCeCCs can have a band gap range of about 2.1 eV to about 2.8 eV. In someembodiments, phosphate post-treated CeCCs can have a band gapdifferential range of about 2.5 eV to about 3.2 eV. Sunlight cansimilarly affect the band gap of a CeCC composition, for example byincreasing a given band gap value by up to about 0.1 eV, up to about 0.2eV, up to about 0.3 eV, up to about 0.4 eV, up to about 0.5 eV, up toabout 0.6 eV, up to about 0.7 eV, up to about 0.8 eV, up to about 0.9eV, or up to about 1.0 eV. In some embodiments the band gap of a CeCC isincreased through Ce(IV) species reduction to Ce(III) species bysunlight in the presence of water. Accordingly, in some embodiments theCeCCs are capable of absorbing light having a range of thresholdwavelengths, for example less than about 590 nm.

While not wanting to be limited to any one photocatalytic watersplitting model, the following proposed model is presented below, whichmay explain the reduction of Ce(IV) into Ce(III) using electrons trappedat the oxygen vacancies while holes are used to oxidize water.

-   -   1. Light with a wavelength (λ)<490 nm is absorbed by the CeCC:        -   CeCC+hv→e⁻ _(CB)+h⁺ _(VB)        -   If nothing else the pair recombines.            -   e⁻ _(CB)+h⁺ _(VB)→recombination→heat    -   2. e⁻ _(CB)+h⁺ _(VB) might get trapped by surface defects:        -   e⁻ _(CB)→e⁻ _(tr)        -   h⁺ _(VB)→h⁺ _(tr)    -   3. Ce⁴⁺ species scavenge away e⁻ _(tr) forming Ce³⁺        -   e⁻ _(tr)+Ce⁴⁺→Ce³⁺    -   4. Simultaneously, the h⁺ _(tr) oxidize adsorbed water        molecules.        -   4 h⁺ _(tr)+2H₂C→4H⁺ _(aq)+O₂(gas)            where CB denotes the conduction band, VB denotes the valence            band, e− denotes an electron, h+ denotes an electron hole,            hv denotes energy or light, tr denotes a trapped hole or            electron, and NHE denotes a normal hydrogen electrode.

Example 1 Forming a CeCC on a Mg Substrate

Forming a CeCC on a Mg substrate includes (A) grinding the substrate toform Layer A, (B) acid cleaning to form Layer B, (C) alkaline cleaningto form Layer C, (D) CeCC deposition to form Layer D, and (E)Phosphating to form Layer E.

Step A: Panels of AZ31B Mg alloy of dimensions 100 mm by 50 mm by 2 mmwere mechanically polished using 180 grit abrasive silicon carbidepapers. The ground surface reacted quickly with the ambient environmentforming a homogenous thin layer A. This oxide/hydroxide layer wasAl-enriched compared to the base AZ31B Mg alloy and containedapproximately equal amounts of oxide and hydroxide compounds. Thenominal elemental surface composition of the AZ31B Mg alloy is given inTable 1, along with the elemental quantification in atomic percent (at.%) of the superficial layers formed at each of the foregoing steps ofthis example.

TABLE 1 Nominal composition of AZ31B Mg Alloy and Summary of ElementalQuantification in Atomic Percent (at. %) of Superficial Layers FormedThereon. Nominal AZ31B Layer A Layer B Layer C Layer D Layer E O 1s N/A57.4 54.1 53.4 69.4 70.5 Mg 2p 95.9-97.4 40.7 38.4 34.4 6.6 7.9 Al 2p2.3-3.2 1.8 7.3 4.6 0.8 1.9 Zn 2p 0.3-0.5 0.1 0.2 0.0 0.0 0.2 Mn 2p0.1-0.5 0 0 0 0 0 Si 2s N/A N/A N/A 7.6 0 0 Ce 3d N/A N/A N/A N/A 23.28.9 P 2p N/A N/A N/A N/A N/A 10.6

The panels were cleaned with isopropyl alcohol, rinsed with deionized(DI) water, and finally dried at room temperature to provide layer A asshown in FIGS. 2A-B. Surface A had relatively homogeneous surfaces withgrooves that were attributed to the grinding (180 grit). Although someareas of contrast were observed, no inclusions or second phases wereexposed on the surface. FIG. 2A shows a cross section of the surfacepresented in FIG. 2B. A homogeneous layer about 50 nm thick formed onthe surface of the AZ31B Mg alloy during the grinding step (layer A).Chemical analysis of the surface performed with a Kratos Axis 165 X-rayphotoelectron spectrometer (XPS) indicated the presence of Mg, Al, Zn, Oand C, shown in Table 1. The uncorrected Mg and Al contents of 40.7 at.% and 1.8 at. %, respectively, are close to the nominal correctedcomposition of AZ31B. However, layer A was slightly Al enriched sincethe Mg/Al at. % ratio calculated from Table 1 was about 23 and theexpected ratio for the nominal composition is in the range of 29 to 42.Layer A was found to be about 59 at. % oxides and about 41 at. %hydroxides by high resolution XPS (HRXPS) measurement of Mg 2p and O 1s. Of the Mg species, oxides accounted for about 58 at. % and hydroxidesaccounted for about 42 at. %.

Step B: Panel surfaces were next pretreated in 1 wt. % HNO₃ aqueoussolution for 30 seconds to provide layer B, which contained bright Alrich particles easily distinguished from the darker Mg matrix as shownin FIG. 3A. Although AZ31B is expected to be a single phase alloy,chemical analysis by energy dispersive X-ray spectroscopy (EDS)confirmed that bright areas were rich in Al and Mn. EDS was performedwith a dual beam Helios NanoLab 600 equipped with an EDS detector toperform chemical analysis coupled with the electron beam for SEMcharacterization and a focused ion beam (FIB) that was used to deposit aprotective Pt layer over the surfaces at each stage of CeCC preparation.The encapsulated surfaces were then milled, thinned and polished toobtain cross-sections. The acid treatment removed the thin nativeoxide/hydroxide layer present after step A forming Layer B, a newhomogeneous layer about 90 nm thick as shown in FIG. 3B. Thecross-sectional image shown in FIG. 3B corresponds to areas where no Alrich particles were visible from the top view. The elementalquantification calculated from the XPS spectrum of layer B is presentedin Table 1. The Mg/Al atomic ratio in layer B was about 5, which wasmuch smaller than the Mg/Al ratio in layer A (about 23). The Alenrichment was due to the faster dissolution rate of Mg in acidicsolutions compared to Al. Fitting of Mg 2p and O is peaks showed thatlayer B was found to be about 85 at. % oxides and about 15 at. %hydroxides, by high resolution XPS (HRXPS) measurement of Mg 2p and O 1s. Of the Mg species, oxides accounted for about 74 at. % and hydroxidesaccounted for about 26 at. %.

Step C: Panel surfaces were next cleaned in an alkaline aqueous solutioncontaining 5 wt. % of Na₂SiO₃.5H₂O for 5 minutes at room temperature inorder to provide layer C. No Al inclusions were observed after thealkaline treatment and the bright areas observed on the surface, shownin FIG. 4A, were caused by charging effects due to the layer formed onthe surface, not particles of different atomic number. FIG. 4B presentsa cross-section image of the surface shown in FIG. 4A, and shows a newporous layer C. Chemical analysis by EDS confirmed that the porous toplayer was richer in Al than the inner layer. Layers B and C had a totalthickness of about 180 nm. Layer C contained Mg, Al, Si, O and C andquantification results obtained from XPS are presented in Table 1. TheMg/Al atomic ratio for layer C increased to about 8 and can be explainedby the preferential dissolution of Al in alkaline solutions while Mg ispassivated. The Al dissolution may also explain the porosity observed inlayer C. Layer C was found to be about 11 at. % oxides and about 89 at.% hydroxides by high resolution XPS (HRXPS) measurement of Mg 2p and O 1s. Of the Mg species, oxides accounted for about 5 at. % and hydroxidesaccounted for about 95 at. %.

Step D: Panel surfaces were next immersed in an acidic cerium-basedaqueous solution for 120 seconds to provide layer D. The depositionsolution consisted of 4 wt. % of CeCl₃.7H₂O (99.9%, Alfa Aesar), 6.7 wt.% of H₂O₂ (Fisher Chemical, 30 vol %) and 0.25 wt. % of organic gelatin(RDH, Rousselot) in DI water. The deposition solution was prepared bydissolving cerium chloride salt in DI water followed by a pH adjustmentto about 2.1 using HCl. The hydrogen peroxide was added into thesolution a few minutes before deposition. The initial pH of the CeCCdeposition solution was 2.3, with cerium species present as Ce³⁺ ions.In the presence of H₂O₂, the Ce³⁺ ions precipitate as Ce hydroxides orhydrated oxides such as CeO₂.2H₂O. Layer D had a dense morphology andappeared to be well bonded with the Mg surface. The surface of layer Dformed during immersion in the cerium-based solution is shown in FIG.5A. High resolution tunneling electron microscope (HRTEM) images oflayer D showed that nanocrystals of cerium oxide that were <5 nm indiameter were embedded in an amorphous matrix. A cracked surface withsmall Ce-rich spherical particles was observed on the surface of layerD. The cross-section of layer D in a region with no cracks is presentedin FIG. 5B. After CeCC deposition, only two layers were observed, aninner layer, which had a similar appearance to layer B and an outerlayer D. Layer D was the as-deposited CeCC and was about 400 nm thick.

The CeCC layer was deposited onto a surface that initially had theappearance of the previous layer C by partially filling the pores oflayer C and incorporating some of the species into the convertedsurface. Chemical analysis performed by XPS showed that the top surfaceof layer D was mainly composed of Ce and O with other small amounts ofMg, Al, and C as shown in Table 1. The elemental quantificationcalculated from the XPS spectrum of layer D is presented in Table 1.Layer D was found to be about 37 at. % Ce(III) and about 63 at. % Ce(IV)by high resolution XPS (HRXPS) measurement of Ce 3d.

Step E: Following CeCC deposition, the coated panels were post-treatedfor 5 min at 85° C. in a 2.5 wt % NaH₂PO₄ aqueous solution to provide afinal Layer E. FIG. 6A shows the CeCC after phosphate treatment.Post-treatment yielded a dense, homogeneous coating with fewer cracksand smaller nodules than the as-deposited layer D. The homogenoussurface exhibited no significant differences in chemical composition orstructure in any given area as compared to another (i.e., uniform Cecontent and number of nanocrystals per unit area). FIG. 6B illustrates across-sectional image of the post-treated sample and shows Layer E to bea homogeneous and dense structure. The measured thickness of layer Eabout 400 nm, was about the same as layer D. Hence, this was not a newlayer over layer D, but an alteration of layer D due to the phosphatepost-treatment.

The XPS analysis indicated that layer E was mainly composed of Ce, P, O,Mg, Al and Zn as shown in Table 1. Phosphate post-treatment increasedthe amount of Ce(III) species from 37 at. % in Layer D to 47 at. %. Theincrease in Ce(III) species was due to the formation of CePO₄. Layer Econtained a mixture of nanocrystals of cerium dioxide and hydratedcerium phosphate. The partial conversion of the CeCC into phosphatespecies was accompanied by a more spherical nanocrystal morphology withthe nanocrystals more evenly distributed in the amorphous network. Thefinal post-treated CeCC consisted of two layers, an inner layer composedof Mg and Al oxides and an outer layer outer layer containing ceriumdioxide and hydrated cerium phosphate nanocrystals in an amorphousmatrix. Layer E still contained a significant amount of CeO₂, but thecombined presence of Mg oxide and Ce(III) oxide in Layer E decreasedcompared to layer D.

The nanocrystalline structure of the as-deposited CeCC layer D are shownin FIGS. 7A and 7C and the post-treated CeCC layer E in FIGS. 7B and 7D.The structures of layers D and E were characterized by nodularnanocrystals embedded within an amorphous matrix. Lattice imaging of thecurrent structures show electron diffraction patterns consisting ofcontinuous rings with diffuse halos. In addition, most of thecrystalline regions were less than 5 nm in diameter for both coatings.However, the crystalline regions appeared to be more spherical andhomogeneously distributed in the post-treated CeCC as shown in FIGS. 7Band 7C the lattice fringes of layer D were measured giving anapproximate d-spacing of 0.32 nm, which is consistent with the {111}planes of the cubic structure of CeO₂. FIG. 7D corresponding to layer Eshows a smaller d-spacing about 0.28 nm in some of the nanocrystals,which is consistent with the {102} facets of the hexagonal structure ofCePO₄.H₂O. However, some nanocrystals shown in FIG. 7D also exhibitedthe 0.32 nm d-spacing suggesting that CeO₂ crystals were still presentin the CeCC after post-treatment.

FIG. 8 shows the electrochemical impedance spectra of AZ31 Mg alloyspecimen after grinding step A, as-deposited CeCC step D, andpost-treated CeCC step E after 4 hours at open circuit potential (OCP)in 0.05 M NaCl electrolyte. The AZ31 Mg alloy after grinding step Ashows a capacitive loop at high and medium frequencies and an inductiveloop at low frequencies characteristic of bare Mg alloys. The impedancerelated to the capacitive loop of layers D and E exhibited an increaseof about 4× with respect to the uncoated samples which is consistentwith the protective behavior for CeCCs on AZ31Mg alloys. However, at lowfrequencies layers D and E showed different behavior. Layer D had asmall inductive loop inferring that active species penetrated throughthe coating defects while layer E showed another capacitive loopsuggesting a higher resistance surface coating. The difference incorrosion protection of layer E might be related to the reduced numberof cracks after phosphate post-treatment.

Example 2 Photochemical Reduction of Nanostructured Ce(IV) to Ce(III)

A CeCC surface was prepared on a substrate as described in Example 1,and next exposed to sunlight under ambient conditions. Exposure toambient sunlight was performed by covering half of a coated panel withaluminum foil. The partially covered CeCCs were exposed for 18 hours, in6 hour intervals over the course of 3 consecutive days, to directsunlight at a temperature of about 25±5° C. and a relative humidity>65%.

X-ray photoelectron spectra were collected with a Kratos Axis 165 X-rayphotoelectron spectrometer (XPS) using a non-monochromated aluminumX-ray source. A Varian Cary 5 ultraviolet-visible near infraredspectrometer (UV-vis-NIR) in the wavelength range of 310-700 nm was usedto record UV-vis diffuse reflectance spectra and determine opticalbandgaps, E_(g), of the coatings. Surface morphology analysis wasperformed using a Dual Beam Helios NanoLab 600 in scanning electronmicroscopy (SEM) mode. The coatings were about 400 nm thick as measuredby cross-sectional analysis in focused ion beam (FIB) mode.

The as-deposited color of the CeCC changed from yellow to translucentduring sunlight exposure. XPS analysis of the high resolution Ce 3d corelevel spectra revealed that the cerium-based nanocoatings contained amixture of Ce(III) and Ce(IV) species with and without sunlightexposure. The Ce 3d core level spectrum is refined for the spin-orbitsplitting 3d_(5/2) and 3d_(3/2) states. In addition, the 3d_(5/2) and3d_(3/2) states are represented for 5 peaks each indexed as v(index) andu(index), respectively. The initial state of Ce(III) (3d₁₀4f₁) isrelated to the v0, u0, v′, and u′ final states and the initial state ofCe(IV) (3d₁₀4f₀) is related to the v, u, v″, u″, v′″, and u′″ finalstates. The concentration of Ce(III) species calculated from fitting ofthe spectra increased from 44 at. % in unexposed coatings to 57 at. %after sunlight exposure. These results are in agreement with the visualobservations, since the Ce(IV) species are related with the yellowappearance of the unexposed coatings.

FIG. 9 shows the UV-visible spectra of the CeCCs on AZ31B substrateswith and without sunlight exposure. The optical absorption edge of thepart of the panel exposed to sunlight shifted to shorter wavelengthswith respect to the unexposed part of the panel. The cerium-basedcoatings partially absorb light in the visible region as the unexposedpanel absorbs at λ<490 nm and the exposed panel absorbs at λ<460 nm. Theestimated band gaps are 2.5 eV and 2.7 eV for the unexposed and exposedpanels, respectively. The absorbance of the unexposed sample isconsistently higher than the exposed sample across the visible range ofabout 400 nm to about 700 nm.

The morphologies of the CeCCs on AZ31B Mg alloy before and after 18hours of ambient exposure are shown in FIGS. 10A-B, respectively. Themorphology of the CeCC without sunlight exposure shown in FIG. 10Aexhibited a uniform mud-cracked surface morphology with small nodularagglomerates, similar to some CeCCs known in the art. FIG. 10B shows theCeCC surface morphology after being exposed to sunlight as havingsignificantly decreased cracking compared to the unexposed sample. Inaddition, fewer and smaller nodules were observed in thesunlight-exposed sample.

A visible change in color was observed within the first hour of sunlightexposure and the color continued to change until panels that originallyhad a pale yellow color had change to translucent after about 18 hoursof exposure. These observations suggest that photo-assisted reduction ofCe(IV) into Ce(III) species increased as a function of exposure time.The XPS results showed an increase of about 30% in Ce(III) species withsunlight exposure. The changes were mainly detected for the increase ofthe v′, v0, u′, and u0 final state peaks, which are related to thetightly bound Ce(III) electrons in the 4f orbital.

References in the specification to “one embodiment”, “an embodiment”,etc., indicate that the embodiment described may include a particularaspect, feature, structure, moiety, or characteristic, but not everyembodiment necessarily includes that aspect, feature, structure, moiety,or characteristic. Moreover, such phrases may, but do not necessarily,refer to the same embodiment referred to in other portions of thespecification. Further, when a particular aspect, feature, structure,moiety, or characteristic is described in connection with an embodiment,it is within the knowledge of one skilled in the art to affect orconnect such aspect, feature, structure, moiety, or characteristic withother embodiments, whether or not explicitly described.

The singular forms “a,” “an,” and “the” include plural reference unlessthe context clearly dictates otherwise. Thus, for example, a referenceto “a compound” includes a plurality of such compounds, so that acompound X includes a plurality of compounds X. It is further noted thatthe claims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for the use ofexclusive terminology, such as “solely,” “only,” and the like, inconnection with any element described herein, and/or the recitation ofclaim elements or use of “negative” limitations.

The term “and/or” means any one of the items, any combination of theitems, or all of the items with which this term is associated. Thephrase “one or more” is readily understood by one of skill in the art,particularly when read in context of its usage. For example, one or morechannels on a microchip can refer to about 1 to about 100 channels, orabout 100 to about 1000 channels.

The term “about” can refer to a variation of ±5%, ±10%, ±20%, or ±25% ofthe value specified. For example, “about 50” percent can in someembodiments carry a variation from 45 to 55 percent. For integer ranges,the term “about” can include one or two integers greater than and/orless than a recited integer at each end of the range. Unless indicatedotherwise herein, the term “about” is intended to include values, e.g.,weight percentages, proximate to the recited range that are equivalentin terms of the functionality of the individual ingredient, thecomposition, or the embodiment. The term about can also modify theend-points of a recited range as discuss above in this paragraph.

As will be understood by the skilled artisan, all numbers, includingthose expressing quantities of ingredients, properties such as molecularweight, reaction conditions, and so forth, are approximations and areunderstood as being optionally modified in all instances by the term“about.” These values can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings of the descriptions herein. It is also understood that suchvalues inherently contain variability necessarily resulting from thestandard deviations found in their respective testing measurements.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges recited herein also encompass any and all possible sub-ranges andcombinations of sub-ranges thereof, as well as the individual valuesmaking up the range, particularly integer values. A recited range (e.g.,weight percentages or carbon groups) includes each specific value,integer, decimal, or identity within the range. Any listed range can beeasily recognized as sufficiently describing and enabling the same rangebeing broken down into at least equal halves, thirds, quarters, fifths,or tenths. As a non-limiting example, each range discussed herein can bereadily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art, all languagesuch as “up to”, “at least”, “greater than”, “less than”, “more than”,“or more”, and the like, include the number recited and such terms referto ranges that can be subsequently broken down into sub-ranges asdiscussed above. In the same manner, all ratios recited herein alsoinclude all sub-ratios falling within the broader ratio. Accordingly,specific values recited for radicals, substituents, and ranges, are forillustration only; they do not exclude other defined values or othervalues within defined ranges for radicals and substituents.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, theinvention encompasses not only the entire group listed as a whole, buteach member of the group individually and all possible subgroups of themain group. Additionally, for all purposes, the invention encompassesnot only the main group, but also the main group absent one or more ofthe group members. The invention therefore envisages the explicitexclusion of any one or more of members of a recited group. Accordingly,provisos may apply to any of the disclosed categories or embodimentswhereby any one or more of the recited elements, species, orembodiments, may be excluded from such categories or embodiments, forexample, for use in an explicit negative limitation.

What is claimed is:
 1. A method for producing a composition having acerium-containing conversion coating on a substrate, the methodcomprising: immersing a metal substrate in a cerium-containing aqueousbath; spontaneously depositing a nano-coating on the substrate, whereinthe nano-coating includes: an amorphous matrix including one or more ofcerium oxide, cerium hydroxide, and cerium phosphate; and crystallineregions including one or more of crystalline cerium oxide andcrystalline cerium hydroxide; and immersing the nano-coated substrate ina phosphate-containing solution sufficient to convert the depositednano-coating to a phosphated nano-coating including: an amorphous matrixincluding one or more of cerium oxide, cerium hydroxide, and ceriumphosphate; and crystalline regions including one or more of crystallinecerium oxide, crystalline cerium hydroxide, and crystalline ceriumphosphate.
 2. The method of claim 1, further comprising preparing thesubstrate prior to immersion by one or more of grinding, acid treatment,and alkaline treatment.
 3. The method of claim 1, wherein the substratecomprises one or more of aluminum or magnesium.
 4. The method of claim3, wherein the substrate comprises one of an AZ31 alloy, an AZ61 alloy,an AZ91 alloy, an AM30 alloy, an AM60 alloy, an AA 7075-T6 alloy, an AA2024-T3 alloy, and Al-clad alloys.
 5. The method of claim 1, wherein theaqueous bath further comprises one or more of an accelerator andanti-bubbling agent.
 6. The method of claim 1, wherein the diameter ofeach crystalline region of the phosphated nano-coating is less than 50nanometers.
 7. The method of claim 2, wherein preparing the substratecomprises surface grinding followed by acid treatment and then followedby alkaline immersion cleaning.
 8. The method of claim 2, whereinpreparing the substrate comprises acid treatment followed by alkalineimmersion cleaning.
 9. The method of claim 2, wherein acid treatmentcomprises treating using one or more of sulfuric acid, nitric acid, andhydrofluoric acid.
 10. The method of claim 1, wherein thecerium-containing aqueous bath comprises about 0.1 wt. % to about 2.0wt. % elemental cerium.
 11. The method of claim 1, wherein thecerium-containing aqueous bath comprises one or more of CeCl₃.7H₂O andCe(NO₃)₃.6H₂O as cerium sources.
 12. The method of claim 1, where thecerium-containing aqueous bath comprises one or more of Ce(III) andCe(IV) species.
 13. The method of claim 5, wherein the acceleratorcomprises an organic accelerator.
 14. The method of claim 5, wherein theanti-bubbling agent comprises one or more of organic and syntheticgelatins.
 15. The method of claim 5, wherein the anti-bubbling agent ispresent in amounts from about 10 ppm to about 1,000 ppm.
 16. A methodfor producing a composition having a cerium-containing conversioncoating on a substrate, the method comprising: immersing a metalsubstrate in a cerium-containing aqueous bath; and spontaneouslydepositing a nano-coating on the substrate, wherein the nano-coatingincludes: an amorphous matrix including one or more of cerium oxide,cerium hydroxide, and cerium phosphate; and crystalline regionsincluding one or more of crystalline cerium oxide and crystalline ceriumhydroxide.
 17. The method of claim 16, further comprising preparing thesubstrate prior to immersion by one or more of grinding, acid treatment,and alkaline treatment.
 18. The method of claim 16, wherein thesubstrate comprises one or more of aluminum or magnesium.
 19. The methodof claim 16, wherein the aqueous bath further comprises one or more ofan accelerator and anti-bubbling agent.
 20. The method of claim 17,wherein preparing the substrate comprises acid treatment followed byalkaline immersion cleaning.